A major concern in the transfusion of donated, stored whole human blood or the various blood cells or protein fractions isolated from whole blood is the possibility of viral contamination. Of particular concern are the blood-borne viruses that cause hepatitis (especially hepatitis A, hepatitis B, and hepatitis C) and acquired immune deficiency syndrome (AIDS). While any number of cell washing protocols may reduce the viral contamination load for samples of blood cells, by physical elution of the much smaller virus particles, such washing alone is insufficient to reduce viral contamination to safe levels. In fact, some viruses are believed to be cell-associated, and unlikely to be removed by extensive washing and centrifugal pelleting of the cells. Current theory suggests that safe levels will ultimately require at least a 6 log (6 orders of magnitude) demonstrated reduction in infectious viral titer for cellular blood components. This 6 log threshold may be greater for plasma, protein components, especially the clotting factors (Factor VIII, Factor IX) that are administered throughout the life of some hemophilia patients.
All blood collected in the United States is now screened for six infectious agents: HIV-1, HIV-2, HTLV-1, hepatitis B virus, hepatitis C virus and syphilis. Additionally, donors are screened for risk factors, and potential donors are eliminated that are considered at risk for the HIV virus. Despite these measures, the risk of becoming infected by a potentially deadly virus or bacteria via the transfusion of blood or blood products remains serious. Screens for contaminants are by nature not foolproof. There is also the quite likely occurrence of new infectious agents that enter the blood supply before the significance of the event is known. For example, by the end of June 1992, the Center for Disease Control reports that 4,959 AIDS cases could be traced to the receipt of blood transfusions, blood components or tissue.
Viral inactivation by stringent sterilization is not acceptable since this could also destroy the functional components of the blood, particularly the erythrocytes (red blood cells) and thrombocytes (platelets) and the labile plasma proteins, such as clotting factor VIII. Viable RBC's can be characterized by one or more of the following: capability of synthesizing ATP; cell morphology; P.sub.50 values; filterability or deformability; oxyhemoglobin, methemoglobin and hemochrome values; MCV, MCH, and MCHC values; cell enzyme activity; and in vivo survival. Thus, if virally inactivated cells are damaged to the extent that the cells are not capable of metabolizing or synthesizing ATP, or the cell circulation is compromised, then their utility in transfusion medicine is compromised.
Viral inactivation by stringent steam sterilization is not acceptable since this also destroys the functional components of the blood, particularly the blood cells and plasma proteins. Dry heat sterilization, like wet steam, is harmful to blood cells and blood proteins at the levels needed to reduce viral infectivity. Use of stabilizing agents such as carbohydrates does not provide sufficient protection to the delicate blood cells and proteins from the general effects of exposure to high temperature and pressure.
Methods that are currently employed with purified plasma protein fractions, often followed by lyophilization of the protein preparation, include treatment with organic solvents and heat or extraction with detergents to disrupt the lipid coat of membrane enveloped viruses. Lyophilization (freeze-drying) alone has not proven sufficient to inactivate viruses, or to render blood proteins sufficiently stable to the effects of heat sterilization. The organic solvent or detergent method employed with purified blood proteins cannot be used with blood cells as these chemicals destroy the lipid membrane that surrounds the cells.
Another viral inactivation approach for plasma proteins first demonstrated in 1958 has involved the use of a chemical compound, beta-propiolactone, with ultraviolet (UV) irradiation. This method has not found acceptance in the United States due to concern over the toxicity of beta-propiolactone in the amounts used to achieve some demonstrable viral inactivation and also due to unacceptable levels of damage to the proteins caused by the chemical agents. Concern has also been raised over the explosive potential for beta-propiolactone as well.
There is significant interest in an effective viral inactivation treatment for human blood components, which will not damage the valuable blood cells or proteins. The treatment must be nontoxic and selective for viruses, while allowing the intermingled blood cells or proteins to survive unharmed.
There is an immediate need to develop protocols for the inactivation of viruses that can be present in the human red blood cell supply. For example, only recently has a test been developed for Non A, Non B hepatitis, but such screening methods, while reducing the incidence of viral transmission, do not make the blood supply completely safe or virus free. Current statistics indicate that the transfusion risk per unit of transfused blood is as high as 1:3,000 for Non A, Non B hepatitis (hepatitis C), and ranges from 1:60,000 to 1:225,000 for HIV, depending on geographic location. Clearly, it is desirable to develop a method which inactivates or removes virus indiscriminately from the blood.
Contamination problems also exist for blood plasma protein fractions, such as plasma fractions containing immune globulins and clotting factors. For example, new cases of non A, non B hepatitis and hepatitis A have occurred in hemophilia patients receiving protein fractions containing Factor VIII which have been treated for viral inactivation according to approved methods. Therefore, there is a need for improved viral inactivation treatment of blood protein fractions.
The ability to inactivate bacterial contaminants from blood and blood products may be as critical as reducing viral contaminants. Between 1986 and 1991, the Food and Drug Administration reported that 15.9% of all transfusion related fatalities were associated with the transfusion of bacterially contaminated blood components. Most of these fatalities were due to the transfusion of bacterially contaminated platelets.
Psoralens are naturally occurring compounds which have been used therapeutically for millennia in Asia and Africa. The action of psoralens and light has been used to treat vitiligo and psoriasis (PUVA therapy; Psoralen Ultra Violet A) and more recently various forms of lymphoma.
Psoralen will bind to nucleic acid double helices by intercalation between base pairs; adenine, guanine, cytosine and thymine (DNA) or uracil (RNA). Upon absorption of UVA photons the psoralen excited state has been shown to react with a thymine or uracil double bond and covalently attach to both strands of a nucleic acid helix.
The crosslinking reaction is specific for a thymine (DNA) or uracil (RNA) base and will proceed only if the psoralen is intercalated in a site containing thymine or uracil. The initial photoadduct can absorb a second UVA photon and react with a second thymine or uracil on the opposing strand of the double helix to crosslink the two strands of the double helix. ##STR1##
Lethal damage to a cell or virus occurs when a psoralen intercalated into a nucleic acid duplex in sites containing two thymines (or uracils) on opposing strands sequentially absorb 2 UVA photons. This is an inefficient process because two low probability events are required, the localization of the psoralen into sites with two thymines (or uracils) present and its sequential absorption of 2 UVA photons.
U.S. Pat. No. 4,748,120 of Wiesehan is an example of the use of certain substituted psoralens by a photochemical decontamination process for the treatment of blood or blood products. The psoralens described for use in the process do not include halogenated psoralens, or psoralens with non-hydrogen binding ionic substituents. Using traditional psoralens such as 8-MOP, AMT and HMT, it is imperative that additives be added into the blood product solution in conjunction with UV irradiation in order to scavenge singlet oxygen and other highly reactive oxygen species formed by irradiation of the psoralen. Without the addition of reactive oxygen species scavengers, cellular components and protein components in the blood product are seriously damaged upon irradiation. (See also, U.S. Pat. No. 5,176,921.) It is clear, therefore, that irradiation of psoralens such as 8-MOP and AMT in aqueous solution creates a competition between the inefficient photocrosslinking reaction and the generation of highly reactive oxygen species. It is also possible that much of the viral deactivation seen using these photosensitizers actually results from the action of the reactive oxygen species against the viral contaminants rather than the inefficient photocrosslinking mechanism.
Whenever photoactivated techniques are relied on in any synthetic or deactivation process, it is critical to remember that it is extremely rare when the introduction of energy into a molecular chromophore results in a single chemical pathway for dissipation of the energy. The result of this dilemma in this particular area of research is 1) that the mechanism for viral inactivation can take a variety of pathways, and 2) that unwarranted and unanticipated side reactions may occur.
As described in previous applications commonly owned with this application, some of the best compounds for use in photo-assisted viral inactivation have a psoralen or coumarin backbone. These compounds are nucleic acid intercalators, and thus assure that the photosensitizer associates with the nucleic acid of viral or bacterial contaminants in biological solutions. Unfortunately, it has been shown that the use of psoralen and coumarin photosensitizers can give rise to alternative chemical pathways for dissipation of the excited state that are either not beneficial to the goal of viral inactivation, or that are actually detrimental to the process. For psoralens and coumarins, this chemical pathway is likely to lead to the formation of a variety of ring-opened species. FIG. 1 depicts the possible structure of several ring-opened species that may result from the inactivation of a coumarin photosensitizer. Similar types of ring-opened species can be envisioned as arising from the inactivation of psoralen backbone photosensitizers.
The formation of non-productive reaction products upon irradiation leads to two concerns. The first concern is related to the toxicity of byproducts formed, and the second concern is the further reactivity of such byproducts. Additional reactions to the byproducts can either be photo assisted or simply due to the highly reactive nature of the compounds.
Considering the complexity of a system that includes, for example, red blood cells and a photosensitizer that is irradiated, it is naturally impossible to pinpoint all the modes of reaction that will lead to successful viral inactivation as opposed to detrimental surface modification of the red blood cells. Researchers in this field have typically been guilty of over-simplifying the claimed mechanisms involved in such a complex system. Many in the field believe that psoralen irradiation leads to viral irradiation strictly by the photoactivating mechanism described above. Others have speculated that the formation of highly reactive oxygen species such as singlet oxygen actually is the species that leads to both viral inactivation and cell surface modification.
The complexity of these systems is only recently being fully appreciated by researches searching utilizing photosensitizers for viral inactivation processes:
Unfortunately it is particularly rare to find photo chemical agents that undergo a single type of chemical reaction. Nearly all photosensitizers that produce singlet oxygen are capable, at least in principle, of undergoing a number of other types of reactions, such as direct attack of the sensitizer-excited states on substrates or formation of radicals, especially in the complex mixtures of compounds characteristic of living systems. (Chapter 4 in "Psoralen DNA Photobiology", Vol. II, F. P. Gasparro, ed. 1988). PA1 1) The intent of this line of research was to improve the poor water solubility of the basic psoralen nucleus. PA1 2) None of the psoralens described were halogenated as are the photosensitizers of the present invention. PA1 4) Later conducted studies showed that a cationic group on a large linker, when added to the 5 or 8 position on the psoralen ring, gave the psoralen nucleus improved binding with native DNA relative to corresponding 5-MOP and 8-MOP analogues. PA1 5) Sidechain substitution at the 5 position was found to be less desirable then substitution at the 8 position. PA1 6) A study of 5-aminomethyl derivatives of 8-MOP showed that most of the amino compounds had a much lower ability to both photobind and form crosslinks to DNA compared to 8-MOP. These reports actually suggest that the primary amino functionality is the preferred ionic species for both photobinding and crosslinking.
Despite this recognition, and the obvious fact that the nature of the causative agents that damage cells and proteins during irradiation is not known, the prior art tends to focus on singlet oxygen and the use of antioxidant quenchers. What is needed is a "blocking" agent that will serve to block deleterious cell or protein damage independent of the claimed reaction that leads to the damage.
U.S. patent application Ser. Nos. 07/510,234 filed Apr. 16, 1990 and 07/686,334 filed Apr. 16, 1991--both incorporated herein by this reference--describe a novel photosensitizer utilizing the quinolone backbone. It is speculated that the use of photosensitizers with the quinoline or quinolone backbone will be less susceptible to ring opening side reactions upon irradiation.
Attempts to inactivate viral decontaminants using photosensitizers and light have also been developed using some non-psoralen photosensitizers. The photosensitizers that have been employed are typically dyes. Examples include dihematoporphyrin ether (DHE), Merocyanine 540 (MC540) and methylene blue.
In any event, an effective radiation photosensitizer must bind specifically to nucleic acids and must not accumulate in significant amounts in lipid bilayers, which are common to viruses, erythrocytes, and platelets. Although there is evidence that psoralens bind to nucleic acids by intercalation, neutral psoralens such as 8-MOP (8-methoxypsoralen) are uncharged and thus also have a high affinity for the interior of lipid bilayers and cell membranes. ##STR2##
The binding of 8-MOP to cell membranes, shown above, would be acceptable if the psoralen bound to the lipid was photochemically inert. However, Midden (W. R. Midden, Psoralen DNA photobiology, Vol II (ed. F. P. Gaspalloco) CRC press, pp. 1. (1988) has presented evidence that psoralens photoreact with unsaturated lipids and photoreact with molecular oxygen to produce active oxygen species such as superoxide and singlet oxygen that cause lethal damage to membranes. Thus, it is believed that 8-MOP is an unacceptable photosensitizer because it sensitizes indiscriminate damage to both cells and viruses.
Positively charged psoralens such as AMT (4'-aminomethyl-4,5',8-trimethylpsoralen) will not bind to the interior of phospholipid bilayers (membranes) because of the presence of the charge. However, AMT contains an acidic hydrogen which can bind to the phospholipid head group by hydrogen bonding, shown below. ##STR3##
Thus AMT is believed to be an unacceptable photosensitizer because it will indiscriminately sensitize damage to viral membranes and to the membranes of erythrocytes and platelets.
Studies of the affects of cationic sidechains on furocoumarins as photosensitizers are reviewed in Psoralen DNA Photobiology, Vol. I, ed. F. Gaspano, CRC Press, Inc., Boca Raton, Fla., Chapter 2. The following points can be gleaned from this review:
U.S. Pat. No. 5,216,176 of Heindel describes a large number of psoralens and coumarins that have some effectiveness as photoactivated inhibitors of epidermal growth factor. Included among the vast functionalities that could be included in the psoralen or coumarin backbone were halogens and amines. The inventors did not recognize the significance of either functionality or the benefits of a photosensitizer including both functionalities.
U.S. patent application Ser. Nos. 08/165,305 and 08/091,674 are commonly assigned with the present application, and are parent applications to this application. These applications disclose the use of a novel class of psoralen photosensitizers that are superior for use with irradiation to inactivate viral and bacterial contaminants in blood and blood products. The psoralens are characterized by the presence of a halogen substituent and a non-hydrogen binding ionic substituent to the basic psoralen side chain. See also, Goodrich et al. Proc. Natl. Acad. Sci. USA, 91:5552-56 (1994).